A fiber optic gyroscope is a navigation sensor that is based on comparative phase measurements of electromagnetic radiation traveling over a known distance of optical waveguide materials. Fiber optic gyroscopes use a wound, circular, optical fiber bundle to guide and transmit light waves, that are the physical basis for measuring co-planar, co-axial rotational movement. More specifically, the optical fibers provide a time-dependent, optical output signal to indirectly determine the amount of rotation by an object (e.g. an aircraft) about an axis of rotation in a plane. One particular type of fiber optic gyroscope is a Sagnac interferometer, which includes a light source that provides an optical signal, a multi-turn coil of optical fibers (hereafter referred to as a fiber optic sensor coil), detectors, and electronic read-out and control circuits.
The fiber optic gyroscope includes at least one fiber optic sensor coil that is formed by tightly winding the optical fibers into a cylindrical structure. In most instances, a typical fiber optic gyroscope is comprised of three fiber optic sensor coils positioned in a three-axis, independent orthogonal configuration to sense rotation about each of three respective orthogonal axes. The gyro's optical fiber is typically comprised of an inner glass core, an outer glass cladding layer and a thin, polymeric coating (e.g. acrylic-based) placed over the glass core. The inner glass and the outer glass are fabricated to have different optical indices of refraction. The optical fiber is typically bound to itself through the use of an adhesive coating that is applied to the full length of the fiber surface as the fiber is wound into a coil. These fiber optic sensor coils are formed of thousands of meters of optical glass fiber that is used to transmit optical signals over long distances with low loss and distortion.
During fabrication of the fiber optic sensor coils, the optical fibers are tightly wound into a cylindrical structure, such that the optical fiber is bound to itself through the use of an adhesive coating. Any unplanned gaps or separations that are formed between the optical fibers during the winding process, impart unacceptable sensor sensitivity to small changes in ambient pressure. In addition, during fabrication a fully insulating overcoat is formed on the external surface of the optical sensor coil. This overcoat, typically formed of a polymer, acts as a plastic barrier restraining the free flow of gasses entrapped in the sensor coil from escaping, thus contributing to gap formation. The sensitivity to changes in ambient pressure is due to high frequency vibrations of loose sub-surface fiber strands in the gaps or separations formed during the winding and overcoat processes for the optical fiber used to form the sensor coil. Free-standing fiber strands in the fiber optic sensor coil can also be the result of thermal or mechanical shock induced by the surrounding environment Such vibration induces a random phase change in the traversing light signal. This phase change is noise in the true, baseline signal and the affected navigation sensor produces erroneous data.
Thus, there is a need for a fiber optic sensor coil and method of fabricating a fiber optic sensor coil for use in a fiber optic gyroscope whereby the sensor coil is not susceptible to small changes in ambient pressure and thereby able to produce more accurate coil rotation information.